Because of the generally recognized risk of climate change, there is worldwide interest in reducing the emission of greenhouse gases, in particular of CO2 (carbon dioxide). So-called CCS (carbon capture and storage) or CO2 sequestration, that is to say the separation of the CO2 from the thermal power process in a power plant and storage of the separated CO2 separately from the atmosphere, is considered to be a realistic step for reducing the CO2 emission into the atmosphere within a relatively short time. CO2 separation is carried out either from the flue gases after the combustion of a fuel containing carbon or by a chemical reaction, in which the carbon is separated from the fuel before combustion. The CCS process includes regeneration of absorbers, adsorbers, or other CO2 separators.
Separation of CO2 after combustion, also referred to as backend capture or post combustion capture, is one of the most promising CCS technologies, and can also be used in particular for combined-cycle power plants.
All known CCS technologies require a relatively large amount of power. In the case of backend capture, the specific energy consumption per kilogram of separated CO2 is inversely proportional to the CO2 concentration in the flue gases from which the CO2 is separated. In order to separate CO2 from gas flows with a relatively low CO2 content, as is the case in the flue gas from conventional gas turbine power plants or combined-cycle power plants (or gas turbine power plants with power/heat coupling), a relatively large amount of energy is correspondingly required per kilogram of separated CO2. The CO2 content of the flue gases depends on the nature of the gas turbine, the fuel gas used and the operating point of the gas turbine. Various concepts have been proposed to increase the CO2 concentration in the flue gases, and therefore to increase the efficiency of the separation process. One concept is flue gas recirculation, for example as has been proposed by O. Bolland and S. Sæther in “NEW CONCEPTS OF NATURAL GAS FIRED POWER PLANTS WHICH SIMPLIFY THE RECOVERY OF CARBON DIOXIDE” (Energy Conyers. Mgmt Vol. 33, No. 5-8, pp. 467-475, 1992). Another concept is the series connection of CCPP, as has been proposed, for example, in US2008/0060346. In this case, the flue gases from a first CCPP are cooled down, and are used as inlet gases for a second CCPP. The flue gases from the second CCPP are cooled and are then passed on for CO2 separation. This virtually doubles the CO2 content of the flue gases, halves the total mass flow of flue gas to be treated, and correspondingly reduces the required plant size and energy consumption.
Furthermore, it has already been proposed in the past for the CO2 absorption to be carried out at an increased pressure. Because of the reduced volume flows, this makes it possible to reduce the plant size and to exploit an increased capacity of the absorber at an increased pressure. For this purpose, WO 00/48709 proposes re-compression of the flue gases for absorption, with subsequent heating and expansion in a turbine. Since the energy for CO2 absorption at high pressure is inversely proportional to the CO2 partial pressure, the energy consumption resulting from absorption at an increased pressure could ideally lead to an efficiency improvement. Since the additional compression results in unavoidable losses, at least some part of the positive effect is, however, lost.